Pipe fittings and, more specifically, hollow, tubular shaped pipe fittings having connector ends with either internal taper threads (commonly referred to as female pipe threads) or external taper threads (commonly referred to as male pipe threads) are commonly used in the plumbing and hardware industry. Typically, male pipe threads of a pipe fitting are threaded into female pipe threads in a second pipe fitting to form a pipe joint. The pipe joint relies upon the wedging action of the oppositely tapered male and female pipe threads to seal the pipe joint. In operation, the wedging action between the male and female pipe threads causes the pipe fitting with the male pipe threads to compress and the pipe fitting with the female pipe threads to expand. However, the wedging action can cause structural failure of the female pipe fitting during joint make-up if the pipe joint is overtightened or stress fatigue failure of the female pipe fitting over time, since the tubular shape of the pipe fitting is more susceptible to expansion than compression.
Pipe fittings made of plastic with female pipe threads are particularly susceptible to structural failure. These plastic pipe fittings can fail prior to obtaining a pressure-tight seal at the pipe joint. Thus, an entire pipe system may have to be derated as a result of the weakened plastic pipe joint.
The prior art has attempted to solve this problem by placing a reinforcement band around the outside of the female pipe fitting to restrict and/or inhibit expansion and splitting of the pipe fitting. However, this method has proved not to be satisfactory. Accordingly, there is a need for strengthened pipe fittings with female pipe threads and a method for making pipe fittings with female pipe threads which (i) withstand deformation produced during joint make-up, overtightening of the pipe joint and stress fatigue failure over time, and (ii) allow for the use of plastic pipe fittings without the need for pressure derating in the pipe system.
In U.S. Pat. No. 5,582,439, the entire contents of which are incorporated by this reference, a strengthened pipe fitting is disclosed which solves most of the aforementioned problems. The strengthened pipe fitting comprises a pipe fitting having an internally threaded surface initiated from an attachment end and a compression band fitted onto the attachment end. The pipe fitting is initially manufactured in the absence of the compression band, so as to have an internally threaded surface with a pitched diameter larger than that specified for mating with a desired externally threaded member. The compression band is then fitted onto the attachment end of the pipe fitting. The compression band is uniquely sized to compress the internally threaded surface sufficiently so that the internally threaded surface mates with the externally threaded member. Since the compression band compresses the attachment end of the pipe fitting, the pipe fitting is subjected to a compressive preload. Thus, the expansion load upon the internally threaded surface resulting from wrench-tight engagement with the externally threaded member is reduced by the preexisting compressive load subjected upon the strengthened pipe fitting.
Unfortunately, it has been found that this unique strengthened pipe fitting is not wholly without problems. Specifically, a problem has been found when using this strengthened pipe fitting in corrosive atmospheres. Because the compression band is most typically made of a metal, use of the strengthened pipe fitting within a corrosive atmosphere results in the corrosion, and sometimes failure, of the compression band.
Accordingly, there is a need for a metal or plastic fitting that overcomes this problem in the prior art.